05000277/LER-2001-005
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2772001005R00 - NRC Website | |
Unit Conditions Prior to the Event The condition was discovered on 10/30/01 with Unit 2 in Mode 1 (Run) at 50% reactor power. The reportable condition occurred on 10/27/01 at 1801 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.852805e-4 months <br /> when the unit entered Mode 2.
Prior to discovering the condition, the unit was in Mode 4 (cold Shutdown) at 0% reactor power and preparations were underway for restart following forced maintenance outage 2F0102.
Description of the Event
On 10/27/01 at approximately 1801 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.852805e-4 months <br /> a condition prohibited by Technical Specifications occurred when Unit 2 was placed in Mode 2 without all four emergency diesel generators (EDG)(EIIS:EK) being operable.
This event was discovered on 10/30/01 at 0159 hours0.00184 days <br />0.0442 hours <br />2.628968e-4 weeks <br />6.04995e-5 months <br /> when the E-2 EDG was undergoing testing in accordance with procedure ST-0-052-122-2 "E-2 Diesel Generator RHR Pump Reject Test". During the test, the diesel generator was being loaded onto the E-22 Bus and an alarm was received for "Jacket Coolant Low Pressure". Minutes later the alarm for the "AUX Jacket Coolant Pump" was received and the Auxiliary Jacket Coolant Pump started. After a brief period, the diesel tripped on low jacket coolant pressure. This trip is a protective feature of the diesel control logic and functioned as designed.
The cause of the trip was promptly investigated and a walkdown of the alignment of the jacket and air coolant loops revealed that manual valve HV-0-52E-10025B, "E-2 D/G Coolant Expansion Tank 0BT097 Gravity Feed Block Valve" (EIIS:V),was in the closed position. This valve is required to be in the open position.
Engineering evaluation demonstrated that the subject valve has to be open in order for the EDG to be operable. Because the E2 EDG was successfully tested on 10/12/01 and the exact time the valve was closed could not be identified, it was determined that EDG was not operable when Unit 2 entered Mode 2 (Startup).
Unit 2 entered mode 2(Startup) from Mode 4 (Cold Shutdown) at 1801 hrs on 10/27/01. In accordance with Technical Specification LCO 3.8.1.c, four diesel generators are required to be operable in Mode 2 to supply the Unit 2 onsite Class lE AC Electrical Power Distribution system. With the E-2 EDG inoperable, entry into Mode 2 without satisfying LCO 3.8.1.c is not permitted by Technical Specification LCO 3.0.4. This LCO prohibits entry into Modes 1, 2 and 3 when an LCO is not met. This LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(i)(B).
Analysis of the Event
Peach Bottom Atomic Power Station Units 2 and 3 have four emergency buses per unit and four shared EDGs. Each of the EDGs has a cooling water system which consists of two cooling loops: the jacket water cooling loop which removes the excess heat of combustion and the air cooler coolant loop which removes the excess heat of compression. A 100 gallon expansion tank provides a positive static suction head for the pumps in each loop as well as maintaining each cooling loop and the associated components full of water. Both of the loops include an engine driven pump and an auxiliary AC motor driven pump, which is provided should the engine driven pump fail.
Each of the cooling loops is monitored by pressure instrumentation which provides alarms and automatically starts the auxiliary pump on low pressure.
If the low pressure condition persists, the diesel control logic will trip the diesel. This protective feature is bypassed under conditions indicative of a loss of coolant accident.
The impact of the valve in the line from the expansion tank being in the closed position was analyzed by station engineering with support from Exelon subject matter experts and the diesel generator manufacturer. The analysis revealed that with the valve closed, both the jacket and air coolant pumps would not have sufficient net positive suction pressure available for proper operation and the E-2 EDG was not operable. This conclusion also encompasses situations where the diesel trip on low pressure is bypassed.
No actual safety consequences occurred as a result of this event. The other three EDGs were not affected and were operable. In addition, the required number of qualified circuits between the offsite transmission network and the Unit 2 onsite Class lE AC Electrical Power Distribution System were operable. If an accident had occurred with the E-2 EDG unavailable, sufficient AC power and equipment were still available to ensure safe shutdown of the plant. Therefore, the actual and potential consequences of this event were minimal.
Cause of the Event
The E-2 EDG was inoperable due to valve HV-0-52E-10025B being in the closed position which isolated the diesel generator coolant expansion tank from the suction of the air and jacket coolant pumps. The valve is located approx 8 ft. above the floor and is beneath the coolant expansion tank in the E-2 bay of the diesel generator building.
A root cause investigation of the incident was performed and entailed a thorough evaluation of maintenance, testing and operations activities that occurred during the period of 10/12/01 through 10/30/01. The successful testing of the E-2 EDG on 10/12/01 provided confirmation that the valve was in the correct position prior to that date. During the testing, the EDG was fully loaded and was run for approximately two hours. Engineering's analysis determined that the test could not have been successfully completed if the valve was closed.
The investigation included consideration of tasks that involved the diesel cooling loops as well as all other known tasks that were performed in the diesel generator building during the period. This was to account for the potential that work was performed on the wrong component or EDG. The investigation included interviews of operations and other station personnel that were in the vicinity of the valve.
Exelon's corporate security organization also investigated the incident and concluded that the closure of the valve was not an intentional act to adversely impact the operation of the EDG.
Based on these investigations, it was determined that the cause of the mispositioning of the valve was due to personnel error that most likely occurred during make-up or draining of coolant from the expansion tank.
Minor coolant leaks in the jacket coolant system existed at the time which required manual action to maintain the level in the tank within prescribed limits. A contributing factor was that the valve was not included in the locked valve program. Given the impact of closure of the valve on the system, combined with the fact that it is infrequently manipulated, the valve should have been controlled as a locked valve.
Corrective Action Completed The valve was reopened. Engineering thoroughly evaluated the operation of the E-2 EDG for the brief period with low coolant pressure and concluded that there was no impact on the machine. Following this, the EDG was successfully tested and declared operable on 10/31/01 at 0535 hours0.00619 days <br />0.149 hours <br />8.845899e-4 weeks <br />2.035675e-4 months <br />.
Corrective Actions Planned Actions are being taken by the station to address the human performance aspects of this event.
To prevent errors from occurring in the future, valve HV-0-52E-10025B will be added to the locked valve program along with the corresponding valves on the other three EDGs.
Previous Similar Occurrences There have been no similar events identified where a valve in the EDG subsystems was incorrectly positioned such that it resulted in the EDG being inoperable.